Electromechanical liquid level servo gauges (ESGs) are used for the accurate measurement of product level and the water interface level in bulk storage tanks used for typical hydrocarbons (often referred to as fuel and oil) as well as for a variety of other liquid chemicals. These products range from very light chemicals, including so-called LPG's (mixtures of propane and butane or even liquefied natural gas (LNG)) to all types of refined products such as naphtha, gasoline, diesel, jet fuels, lubricants and all types of chemicals, both pure and mixed.
The servo principle is based on the measurement of the apparent weight of a displacer that is within the liquid in the tank. The displacer is a mechanical body suspended on a strong thin measuring wire, where the displacer material has a higher density than the liquid to be measured. The measurement wire is wound on a high accuracy machined grooved drum with a calibrated circumference that is coupled to a detection shaft that is coupled to a worm wheel (or servo motor) by ball bearings. The apparent weight resulting from the weight of the displacer minus the weight of the displaced liquid product is measured and is then used by a computing device such as a microcontroller with the servo motor used to rotate drum in order to position the displacer at a different height in the tank.
By rotating the drum the wire is spooled up or paid out into the tank and the displacer is raised or lowered until the measured apparent weight equals the programmed set point. For safety reasons typically a magnetic coupling (using pole pairs) may be located between drum and electronics (motor, microcontroller, electronics, etc.) as many of the liquids products which are commonly stored in bulk storage tanks are flammable and typically need an explosion safe design. The displacer being denser as compared to the density of the liquid product in the tank is basically kept at the same level using Archimedes law which indicates that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces.